Article

Mycorrhiza – a fascinating symbiosis in the forest

Everyone knows about the mushrooms in the forest,
but to most people it is unknown as to why the mushrooms grow mostly in
the forest and which functions they have. A fact sheet issued by the Swiss Federal Research Institute WSL gives insight
into the fascinating community of the mycorrhizae.

The
term "mycorrhiza"
(from Greek mukês for mushroom and rhiza for root) does not mean
anything more than "mushroom-root” or, more elegantly, "mushroomed
root”". A mycorrhiza is a
root which is populated by a mycorrhizal fungus. The fungus covers the
outermost
fine roots with a thick network of strings (so-called mycelium) and
forms a
coat of fungus (Fig. 2).

Approximately one
third of the mushrooms growing in our forests are mycorrhizal fungi. Among these
approximately 2’000 species, many are delicate and edible mushrooms, but there
are toxic ones as well. Many mycorrhizal fungi are host specific, which means
that they only grow with specific tree species. (i.e. Larch Bolete, Oak Milk-cap).
Others only grow in deciduous forests or coniferous forests. In the network of
tree roots, usually several different mycorrhizal fungi live side by side. In Central Europe, mycorrhizae occur on the roots of all tree species.

Fig. 2: The mushrooms form a
thick coat around the outermost fine roots of the tree.
Photo: Simon Egli (WSL)

Functions of the
mycorrhizae

Nutrient exchange:The mycorrhiza is an organ in which substances are exchanged between the tree and the mycorrhizal fungus. While the tree is feeding the fungus with sugar as a product of the photosynthesis, in turn it receives from the fungus several different nutrients such as nitrogen and phosphorus, which the fungus takes up from the finest soil pores with its hyphae (Fig. 3).

Protection from pollutants:Mycorrhizae are able to protect the trees from poisonous effects caused by pollutants. The fungi retain heavy metals which can be taken up by the tree. This characteristic is similar to a filter function. The disadvantage, however, is that these heavy metals are accumulated into the fruiting body of the fungus. This may lead to toxic concentrations within edible mushrooms.

Further functions:Mycorrhizal plants show an increased tolerance towards various stress factors. The trees are therefore less susceptible to frost and gain additional resistance against pathogenic microorganisms in the soil. Furthermore, the mycorrhizae cause an increase in plant growth.

Concrete forest
measures for the stimulation of mycorrhizae

The thinning of dense and dark old stands may stimulate the fruit body production of the mycorrhizal fungi.

The more tree species growing in a forest, the higher the species diversity of mycorrhizal fungi.

After storm damages, the remaining young seedlings are a hideaway for mycorrhizal fungi which have lost their tree partner. They assist in rescuing the fungi into the new tree generation.

Do not burn the harvest which is left over. Leave single dead wood stems behind.

Fig. 3: Mushroom and tree
take advantage of each other through the exchange of nutrients.
Source: WSL